1. The Field of the Invention
The present invention concerns a system and process for controlling the formation of sheet materials such as paper.
2. State of the Art
Various sheet materials are manufactured by causing the material in a fluid state to flow in a controlled fashion onto a conveyer or the like. For example, sheet plastic is often manufactured by extruding heated plastic through a die onto a conveyer belt. Likewise, paper is often manufacture by causing a slurry of paper pulp to flow from a headbox onto a moving wire. In the manufacture of sheet materials, a thickness-regulating member is normally used to insure that the thickness of the sheet is substantially uniform both in the direction in which the sheet travels and in the direction perpendicular thereto. In the case of paper, the thickness regulating member is called a slice lip and in the case of plastics, the thickness regulating member can be called a die. In either case, the position of the thickness-regulating member is controlled by actuators, which in the case of paper manufacturing include slice rods.
U.S. Pat. No. 3,413,192 teaches a system for controlling a thickness regulating member used in the manufacture of sheet products. According to the patent, a water slurry of fibrous paper stock is fed into a headbox, and the slurry then flows through a slice lip opening slot to be deposited in a continuous web onto a Fourdrinier wire which is continuously moving in a direction away from the headbox. The position of the slice lip is controlled by a plurality of actuators connected to the slice lip and to the headbox and spaced apart from one another along the length of the slice lip.
Further, according to the patent, the pape slurry dries as it travels along the Fourdriner wire and thereafter the paper web is fed between press rolls for removal of additional moisture. The web is then fed through a drier section, and the finished paper web issues from the drier. After the dried paper leaves the drier, a conventional basis weight measuring gauge including a source of nuclear radiation is used to measure the thickness of the web across the width thereof. Information from the measuring gauge is transmitted to a control system which in turn controls the slice actuators to maintain the thickness of the paper being produced according to a predetermined scheme.
One of the shortcomings of the system taught in the patent is that the physical characteristics of the slice lip and the actuators are not explicityly considered. It is believed that this leads to certain inaccuracies in the operation of the system.